In general, the non-linear optical effect means a non-linear optical response proportional to the square, cube or higher power of photoelectric field applied. Known examples of the secondary non-linear optical effect proportional to the square of photoelectric field applied include second harmonic generation (SHG), optical rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing and light difference frequency mixing. Examples of the ternary non-linear optical effect include third harmonic generation (THG), optical Kerr effect, self-induced refractive index change and two-photon absorption.
As the non-linear optical material of exhibiting these non-linear optical effects, a large number of inorganic materials have been heretofore found. However, inorganic materials can be hardly used in practice because so-called molecular design so as to optimize desired non-linear optical characteristics or various properties necessary for the production of a device is difficult. On the other hand, organic compounds can realize not only optimization of desired non-linear optical characteristics by the molecular design but also control of other various properties and therefore, the probability of its practical use is high. Thus, organic compounds are attracting attention as a promising non-linear optical material.
In recent years, among non-linear optical characteristics of the organic compound, ternary non-linear optical effects, particularly, non-resonant two-photon absorption and non-resonant two-photon emission, are being taken notice of. The two-photon absorption is a phenomenon such that a compound is excited by simultaneously absorbing two photons. In the case where the two-photon absorption occurs in the energy region having no (linear) absorption band of the compound, this is called resonant two-photon absorption. The non-resonant two-photon emission means emission generated by an excited molecule resultant of the non-resonant two-photon absorption in the radiation inactivation process of the excited state. In the following, even when not particularly specified, two-photon absorption and two-photon emission indicate non-resonant two-photon absorption and non-resonant two-photon emission, respectively.
The non-resonant two-photon absorption efficiency is proportional to the square of photoelectric field applied (square-law characteristic of two-photon absorption). Therefore, when a laser is irradiated on a two-dimensional plane, two-photon absorption takes place only in the position having a high electric field strength at the center part of laser spot and utterly no two-photon absorption occurs in the portion having a weak electric field strength in the periphery. On the other hand, in a three-dimensional space, two-photon absorption occurs only in the region having a large electric field strength at the focus where the laser rays are converged through a lens, and two-photon absorption does not take place at all in the off-focus region because the electric field strength is weak. As compared with the linear absorption where excitation occurs in all positions proportionally to the strength of photoelectric field applied, in the non-resonant two-photon absorption, excitation occurs only at one point inside the space by virtue of the square-law characteristic and therefore, the space resolution is remarkably enhanced. Usually, in the case of inducing non-resonant two-photon absorption, a short pulse laser in the near infrared region having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present, and not having the absorption of the compound is used in many cases. Since a near infrared ray in a so-called transparent region where the (linear) absorption band of a compound is not present is used, the excitation light can reach the inside of a sample without being absorbed or scattered and one point inside the sample can be excited with very high space resolution due to the square-law characteristic of non-resonant two-photon absorption. Therefore, application of the non-resonant two-photon absorption and non-resonant two-photon emission to two-photon contrasting of a living organism or two-photon photodynamic therapy is being expected. Furthermore, when non-resonant two-photon absorption and two-photon emission is used, a photon having an energy higher than the energy of incident photon can be taken out, therefore, in view of a wavelength converting device, studies on up-conversion lasing are also reported. In addition, application of the two-photon absorbing compound to three-dimensional optical recording medium, three-dimensional display, three-dimensional stereolithography and the like is also expected.
As the organic compound of undergoing two-photon emission or up-conversion lasing with good efficiency, so-called stilbazolium derivatives are known (see, G. S. He, et al., Appl. Phys. Lett., 67, 3703 (1995), G. S. He, et al., Appl. Phys. Lett., 67, 2433 (1995), G. S. He, et al., Appl. Phys. Lett., 68, 3549 (1996), G. S. He, et al., J. Appl. Phys., 81, 2529 (1997), P. N. Prasad, et al., Nonlinear Optics, 21, 39 (1999), Y. Ren, et al., J. Mater. Chem., 10, 2025 (2000), G. Zhou, et al., Jpn. J. Appl. Phys., 40, 1250 (2001), and M. Albota, et al., Science, Vol. 281, page 1653 (1998). Furthermore, various applications using the two-photon emission of a stilbazolium compound having a specific structure are described in W097/09043.
In the case of applying the non-resonant two-photon emission to contrasting of a living organism, photodynamic therapy, up-conversion lasing or the like, the two-photon absorption efficiency (two-photon absorbing cross-sectional area) of the organic compound used and the emission efficiency from the excited state resultant of the two-photon absorption must be high. In order to obtain 2-fold two-photon emission intensity by using the same organic compound, 4-fold excitation light intensity is necessary due to the square-law characteristic of two-photon absorption. However, irradiation of excessively strong laser light highly probably incurs, for example, photo-damage of a living organism or photo-deterioration of the two-photon emitting dye itself and therefore, this is not preferred. Accordingly, in order to obtain strong two-photon emission with weak excitation light intensity, an organic compound of efficiently undergoing two-photon absorption and two-photon emission must be developed. The two-photon emission efficiency of stilbazolium derivatives cannot fully satisfy the performance in practical uses.
With recent progress of the advanced information society, a three-dimensional optical recording medium is abruptly attracting an attention as an ultimate high-density, high-capacity recording medium. In the three-dimensional optical recording medium, bit recording is superposed in tens or hundreds of layers in the three-dimensional (thickness) direction to achieve super high-density and super high-capacity recording as large as tens or hundreds of times conventional two-dimensional recording mediums. In order to provide a three-dimensional optical recording medium, access and writing must be performed at an arbitrary position in the three-dimensional (thickness) direction and as a technique therefor, a method of using a two-photon absorbing material is effective.
In the medical field, a three-dimensional display or three-dimensional stereolithography of enabling many persons to observe a natural solid without spectacles is demanded so as to apply a more adequate therapy to a three-dimensionally intricate site such as brain or ear. As a promising material therefor, a three-dimensional volume display or three-dimensional stereolithography composition using two-photon absorption is expected.
However, for practically using the three-dimensional optical recording medium, three-dimensional volume display or stereolithography composition, a high-speed recording technique is necessary. Since the recording speed is proportional to the two-photon absorbing cross-sectional area, if a known two-photon absorbing compound having a low two-photon absorption efficiency is used, a practical material or system cannot be provided. Under these circumstances, development of a compound having a very large two-photon absorbing cross-sectional area is keenly demanded.
As described above, use of non-resonant two-photon absorption and non-resonant two-photon emission enables various applications characterized by very high space resolution. However, two-photon emitting compounds usable at present are low in the two-photon absorbing ability and also bad in the two-photon emission efficiency and therefore, a very high-output laser is necessary as an excitation light source of inducing two-photon absorption and two-photon emission.